1. Field of the Invention
The present invention relates to reducing horizontal banding in inkjet printing, and, more particularly, to an imaging apparatus and method for reducing horizontal banding by determining an optimal print direction for an initial printing pass when generating a printed image with an inkjet printhead.
2. Description of the Related Art
An inkjet print engine of an inkjet printer or multifunction imaging device forms an image on a print media sheet by horizontally scanning one or more inkjet printheads across the print media sheet in multiple printing passes, also referred to as printing swaths, and by indexing the print media sheet in an orthogonal direction, e.g., a vertical direction, between printing passes. Such inkjet print engines are capable of printing in multiple printing modes, e.g., draft, high quality, photo, etc. It is generally recognized that reducing the number of passes of the printheads when printing on plain paper improves the printing throughput compared to the highest quality modes. Also having an effect on print quality is the undesirable generation of horizontal banding. Horizontal banding may include both dark bands and white bands at the boundary between print swaths, where ink drops at the edge of a swath are printed too close or too far from drops in the adjacent or overlapping print swath. Horizontal banding also may include color order differences in different passes, causing darkness and hue differences, as well as dry time differences, both of which may happen inside the print swaths, as well as at the edges.
A scanned banding metric print sample may be used to assess banding visible to the human eye. The banding sample will score differently depending on whether or not there is a mono barcode printed across the top of the sample. The difference the barcode makes is to change the print direction of the first color swath so that all the color swaths are reversed down the banding metric page. The print direction includes whether the color order is cyan-magenta-yellow or yellow-magenta-cyan.
Print quality may further be dependent upon how the ink dots are formed on the printed page. Manufacturing variations contribute to the tendency of both monochrome and color inkjet printheads to show dot quality differences as a function of carrier direction. Each ink drop generated by an inkjet printhead typically includes a mother (primary) drop and at least one satellite drop, wherein a satellite drop typically follows the mother drop. The satellite drop may land on the print medium inside, partially on, or outside the mother drop, and this phenomena is often referred to as satellite asymmetry. Satellite asymmetry is due to a difference in satellite direction with respect to the mother drop, and is very common in manufactured inkjet printheads. It is known that satellite asymmetry can cause graininess of a print recording. Graininess in an image will be aggravated by the presence of satellite dots. One approach, such as that disclosed in U.S. Pat. No. 7,467,843, is to determine the optimal direction of carrier travel in which a printhead exhibits the least tendency to generate unwanted satellites while recording an image to reduce image graininess.